Mels been forming a group (8)
I believe the answer is:
ensemble
'a group' is the definition.
(I know that group can be written as ensemble)
'mels been forming' is the wordplay.
'forming' is an anagram indicator.
'mels'+'been'='melsbeen'
'melsbeen' anagrammed gives 'ENSEMBLE'.
(Other definitions for ensemble that I've seen before include "Musical company" , "team" , "supporting cast" , "Group of musicians provide a full outfit" , "Groups of musicians form a complete outfit" .)